Core B: Patient enrollment, specimen and data management, and biostatistics. This core supports Projects 1 and 2 in four ways. First, the core is responsible for enrolling patients and collecting clinical information that is needed for Projects 1 and 2. Patients are categorized according to the presence of graft- versus-host disease (GVHD) activity and immunosuppressive treatment. This information is obtained primarily by abstracting medical records from providers at the Seattle Cancer Care Alliance and from referring physicians. Second, the core is responsible for procuring blood samples from selected patients who have outcomes that are informative for the development of tolerance after hematopoietic cell transplantation (HCT). For example, samples are routinely obtained from patients when they leave Seattle at approximately 3 months after HCT and when they return for evaluation at 1 year after HCT. In addition, a concerted effort is made to collect blood samples before the onset of immunosuppressive treatment in patients who develop chronic GVHD. Blood samples are also collected from patients before and after infusion of donor lymphocytes for treatment of recurrent malignancy following HCT. In these patients, blood samples are also obtained before immunosuppressive medications are given to treat any GVHD that might occur after donor lymphocyte infusion. These samples are likely to be particularly informative, since donor lymphocyte infusions are used only in patients who have no GVHD after immunosuppressive medications have been withdrawn. In these patients, immune reactions can be monitored with no potential interference from the effects of immunosuppressive medications. Third, the core is responsible for processing blood samples, enumerating cells that express CDS, CD4, CDS, CD25, CD127 and HLA-DR, and freezing samples for future use in Projects 1 and 2. Fourth, the core provides support for data management and biostatistical analysis of results from Projects 1 and 2. RELEVANCE (See instructions): Immune reactions of donor cells against the recipient can cause complications when blood or marrow transplantation is used to treat leukemia and other diseases. The biological mechanisms that control these harmful immune reactions are not well understood. This core strengthens the overall program by providing investigators in Projects 1 and 2 with the clinical information and blood specimens needed for research studies that will help to identify the mechanisms that control these harmful immune reactions.